This invention pertains to large tiled, flat-panel displays and, more particularly, to an apparatus for controlling the temperature and thereby maintaining a bright, uniform illumination in a back light system for large, tiled, flat panel displays with intense lighting capability in the range of 50,000 to 150,000 nits.
Large flat-panel displays made in accordance with known active matrix (or TFT) liquid crystal display technologies are typically mounted in front of a back light module which contains an array of fluorescent lamps. FPDs of this type have been increasing in size by about 1 to 2 inches diagonal yearly. The median size in 1999 for use in desk top PCs is about 15 inches diagonal view area. A few very large displays are made in the range of 20 to 25 inches diagonal. Tiled AMLCD FPDs may be made in the range of 40 inches diagonal, as described in copending U.S. patent application Ser. No. 09/368,921, assigned to the common assignee and hereby included as reference. However, tiling, as described in U.S. Pat. No. 5,661,531 and also included as reference requires extremely intense light sources with substantially collimated lighting, masked optical stacks, and pixel apertures that have very low emitted light efficiency. Thus, lighting with unusually high intensity ranges of 50,000 to 150,000 nits (candellas/square meter) is desirable with uniformity over very large FPD areas. Unique designs, and control features are necessary to achieve such high intensities at reasonable wattages for consumer or business applications.
Maintaining a bright and uniform illumination of the display over its entire active area is very difficult to do. The intensity required for some applications, in particular that required for a large, tiled, flat-panel LCD display as described in U.S. Pat. Nos. 5,661,531, entitled CONSTRUCTION AND SEALING OF TILED, FLAT-PANEL DISPLAYS, and 5,867,236, entitled CONSTRUCTION AND SEALING OF TILES, FLAT-PANEL DISPLAYS, causes the lamps to produce a significant amount of heat in order to meet predetermined brightness specifications. In addition, fluorescent lamps are designed to run most efficiently at predetermined elevated temperature. It is desirable to run them at their ideal design temperature, which is usually between approximately 50 and 60 degrees Centigrade.
Small edge lit back light modules used in notebook or laptop PCs do not produce sufficient brightness for a large area display, nor are they capable of illuminating a large area uniformly. Thus, it is preferable to illuminate the area with an array of fluorescent lamps. The optimum number of lamps required depends on the size of the area to be illuminated and the display brightness specifications preferably in the range of 300 nits to the viewer as described in co-pending patent applications, Ser. Nos. 09/406,977 and 09/407,620 filed concurrently herewith and hereby incorporated by reference. A large area display requires multiple lamps to illuminate it to the desired level properly.
Since most displays are designed to be wider than they are tall, it is advantageous from a reliability and power perspective to use horizontal lamps. This results in fewer lamps and less power, since there are fewer lamp cathodes. The resultant designs have lamp tubes placed horizontally, one above the other. This produces a chimney effect, where the upper lamps receive heated air from the lamps below. The temperature differential from top to bottom can become severe. The lamp tube temperature differences can cause significant variations in the luminance of the back light as well as decreased life expectancy.
This invention provides unique means for controlling the temperatures to be substantially the same for all lamp tubes and allows for maintaining a desired predetermined level of maximum brightness by keeping all of the lamp tubes at or very near optimum temperature. This then provides for uniform brightness at peak efficiencies and the longest possible lamp life. The controls for the back light keep the lamps individually or in groups at the optimum temperature, regardless of the ambient temperature. One component of the control is provided through the combined use of variable speed cooling fans, temperature sensors, and control logic.
This invention further provides a second unique means of predetermined accurate control by the addition of dimming ballasts that can be used in the previously mentioned combined control system to very accurately control the temperature profile in the hierarchy of lamps and add the capability for safety operation (controlled shut down) in the event of gross overheating. With the individual control over lamp input power, each lamp can be dimmed such that it stays at the optimum temperature. This can be done by characterizing the temperature distribution in the back light, or by providing individual sensors on each lamp.
Additionally, the dimming ballasts along with the control logic can be used for keeping the lamps off until the display system is activated. The dimming feature is also used for manual brightness control or ambient light sensing control.
The present invention pertains to back light systems for tiled, flat-panel displays which require unusually high levels of illumination in the range of 50,000 to 150,000 nits. The invention reflects the useful discovery that a substantially constant and uniform luminance output of the back light module is readily obtained if the lamp tube wall temperature is kept substantially constant. This invention provides unique means for achieving this through control of the lamp temperature using a combination of air flow variations and lamp dimming means. Further, the controls can be used in conjunction with ambient light sensing or manual controls to maintain predetermined output light intensity. In addition, the back light module controls can be used to avoid overheating and loss of the display due to such overheating. Another benefit of the control system is enhanced life expectancy, due to minimizing overheating and providing a lower time-averaged lamp temperature.
Methods are disclosed herein for controlling luminance emitted from a back light module for a flat-panel, liquid crystal display (LCD). Fluorescent lamps are commonly used in back light modules for LCDs, due to their high efficiency. Luminance from fluorescent lamps is a function of lamp tube temperature, as is the efficiency and also lamp life. This invention provides means for achieving luminance stability, high efficiency and long life through controlling and maintaining lamp tube wall temperatures. In another embodiment of this invention, means are disclosed to selectively dim the lamps to obtain optimum lamp tube temperatures. Actual practice of this invention has resulted in luminance output over a large area (xc2xd square meter) exceeding 50,000 nits and ability to hold this luminance level within 2 percent over days of continuous operation.
In accordance with the present invention, there is also provided means for safely preventing overheating of the back light and display due to high ambient temperatures or component failures.